feat: improve the image management experience with template builder (#27018)

Makes it easier to pick the right workspace image, both in the template
builder and in the docs.

- Template builder: the Docker and Kubernetes bases now expose a
`container_image` variable in the wizard (freeform text, defaults to
`codercom/example-base:ubuntu`), and their prerequisites explain why
image choice matters, with tradeoffs between
`codercom/example-base:ubuntu` (minimal) and
`codercom/example-universal:ubuntu` (catch-all), plus pointers to
[coder/images](https://github.com/coder/images) and the image management
docs.
- Docs: reworked [image
management](https://coder.com/docs/@ben%2Fdevrel-201-image-guidance-prereqs/admin/templates/managing-templates/image-management)
into a clearer maturity ladder (minimal → golden → project-specific →
developer customization), with pullable image references in every
example, `codercom/oss-dogfood` as a project-specific example, and Dev
Containers + [mise](https://mise.jdx.dev/) as ways to customize without
new images.

Companion PR for the starter templates: coder/registry#943

Part of DEVREL-201.

🤖 Generated with Coder Agents using Claude, on behalf of @bpmct (wizard
variable by @jeremyruppel in #27024)

---------

Co-authored-by: Jeremy Ruppel <jeremyruppel@users.noreply.github.com>

coderd/templatebuilder/testdata/docker.tf.golden

@@ -150,7 +150,7 @@ resource "docker_volume" "home_volume" {
 
 resource "docker_container" "workspace" {
   count = data.coder_workspace.me.start_count
-  image = "codercom/enterprise-base:ubuntu"
+  image = "codercom/example-base:ubuntu"
   # Uses lower() to avoid Docker restriction on container names.
   name = "coder-${data.coder_workspace_owner.me.name}-${lower(data.coder_workspace.me.name)}"
   # Hostname makes the shell more user friendly: coder@my-workspace:~$

coderd/templatebuilder/testdata/kubernetes.tf.golden

@@ -244,7 +244,7 @@ resource "kubernetes_deployment_v1" "main" {
 
         container {
           name              = "dev"
-          image             = "codercom/enterprise-base:ubuntu"
+          image             = "codercom/example-base:ubuntu"
           image_pull_policy = "Always"
           command           = ["sh", "-c", coder_agent.main.init_script]
           security_context {